Field ionization of fluid samples by electronic tunneling due to potential barrier distortions in large electrostatic fields is well known, and numerous structures have been devised for producing the necessary field strength and for feeding fluid to be ionized into the ionizing region. The use of a small diameter tube, or capillary, as one electrode of a field ionization source, through which tube the fluid, such as gas, to be ionized is fed is known and useful when high gas pressures are available. A high electrostatic field necessary for ionization is provided by locating a second electrode adjacent the exit end of the tube, and applying a potential difference across said electrodes. However, as a practical matter, the gas conductance through a small diameter tube having a length that can be reasonably handled is extremely limited. Consequently substantially no use of capillary-type electrodes in field ionization sources has been made.